Method for creating waveguides in multilayer ceramic structures and a waveguide having a core bounded by air channels

ABSTRACT

The invention relates to a waveguide manufacturing and a waveguide manufactured with the method, which can be integrated into a circuit structure manufactured with the multilayer ceramic technique. The core part ( 23, 33, 43, 53   a   , 53   b   , 53   c ) of the waveguide is formed by a unit assembled of ceramic layers, which is limited in the yz plane by two impedance discontinuities and in the xz plane by two planar surfaces ( 24, 25, 34, 35, 54   a   , 54   c   , 55   a   , 55   b   , 55   c ) made of conductive material. The conductive surfaces can be connected to each other by vias made of conductive material ( 38, 39, 48, 49 ). The waveguide manufactured with the method according to the invention is a fixed part of the circuit structure as a whole.

PRIORITY CLAIM

This is a national stage of PCT application No. PCT/FI00/00635, filed onJul. 10, 2000. Priority is claimed on that application, and on patentapplication No. 991585 filed in Finland on Jul. 9, 1999.

FIELD OF THE INVENTION

The invention relates to a method for creating waveguides in circuitboard units manufactured with the multilayer ceramic technique, in whichmethod the dimensions and structural directions of the circuit boardunits can be defined by means of x, y and z axes perpendicular to eachother, and the circuit board unit is assembled of separate ceramiclayers, the permittivity ε_(r) of which is higher than the correspondingvalue of air, and in which layers cavities and holes of the desiredshape can be made, and on the surface of which ceramic layer aconductive material can be printed at the desired location by silkscreen printing, and the circuit board unit is completed by exposing theunit to a high temperature.

The invention also relates to a waveguide integrated into circuit boardunits manufactured with multilayer ceramics, wherein the dimensions andstructural directions of the circuit board units can be defined by meansof x, y and z axes perpendicular to each other, and the circuit boardunit has been assembled of separate ceramic layers, the permittivityε_(r) of which is higher than the corresponding value of air, and inwhich layers cavities and holes of the desired shape have been made inthe ceramic layers, and on the surface of which ceramic layers a layerof conductive material can be added at the desired location by silkscreen printing.

BACKGROUND OF THE INVENTION

Different conductor structures are used in the structures of electronicdevices. The higher the frequencies used in the devices, the greater therequirements set for the conductor structures used, so that theattenuation caused by the conductor structures does not become too highor that the conductor structure used does not disturb other parts of theapparatus by radiation. The designer of the device can select from manypossible conductor structures. Depending on the application, anair-filled waveguide made of metal, for example, can be used. The basicstructure, dimensions, and waveforms that can propagate in the waveguideand the frequency properties of the waveguide are well known (see e.g.chapter 8 Fields and Waves in Communication Electronics, Simon Ramo etal., John Wiley & Sons, inc., USA). FIG. 1 shows, as an example of thedimensioning of a waveguide, a rectangular waveguide made of conductivematerial, the width of which is a in the direction of the x-axis of thecoordinates shown in the figure, the height of which is b in thedirection of the y-axis, and which is filled by air, whose permittivityε_(r) is of magnitude 1. In the air-filled waveguide shown in FIG. 1,the first (lowest) waveform that can propagate in the direction of thez-axis is the so-called TE₁₀(Transverse-electric) waveform. The electricfield E of this waveform does not have a component in the direction ofthe z-axis at all. Instead, the magnetic field H has a component in thedirection of propagation, the direction of the z-axis. The so-calledcut-off frequency f_(c) of the waveform TE₁₀, which means the lowestfrequency that can propagate in the waveguide, is obtained from theequation: $f_{{cTE}_{10}} = \frac{c}{2a}$where the letter a means the width a of the waveguide in the directionof the x-axis, and c is the speed of light in a vacuum. Generally, theusable frequency range of the waveguide is 1.2 to 1.9 times the cut-offfrequency of the waveform in question. The usable lower limitingfrequency is determined by the growth of the attenuation when thecut-off frequency f_(c) is approached from above. The upper frequencylimit again is determined by the fact that with frequencies that aremore than twice the cut-off frequency f_(c) of the desired waveform,other waveforms that are capable of propagating are also created in thewaveguide, and this should be avoided.

There are also known waveguide structures, in which the waveguide isformed by a core part made of dielectric material, which is coated witha thin layer of conductive material. However, these waveguides arealways made as separate components. The above described waveguidestructures provide a small attenuation per unit of length, and they donot emit much interference radiation to the environment. However, theproblem with these waveguides is the large physical size compared to therest of the circuit unit to be manufactured, and the fact that it isdifficult to integrate their manufacture into the manufacture of thecircuit unit as a whole. These waveguides must be joined to the circuitunit mechanically either by soldering or by some other mechanical jointin a separate step, which increases costs and the risk of failure.

Conductor structures that are better integrated into the structure arealso utilized in electronic equipment. These include strip lines,microstrips and coplanar conductors. Their manufacture can be integratedinto the manufacture of the circuit unit as a whole, when circuit unitsare manufactured as ceramic structures. This manufacturing technique iscalled multilayer ceramics, and it is based either on the HTCC (HighTemperature Cofired Ceramics) or LTCC (Low Temperature Cofired Ceramics)technique. The circuit structures implemented with either of thesemanufacturing techniques consist of multiple layers of ceramic material(green tape), which are 100 μm thick and placed on top of each otherwhen the circuit structure is assembled. Before the heat treatment,which is performed as the final treatment, the ceramic material is stillsoft, and thus it is possible to make cavities and vias of the desiredshape in the ceramic layers. It is also possible to make variouselectrically passive elements and the above-mentioned conductors on thedesired points with silk screen printing. When the desired circuit unitis structurally complete, the ceramic multilayer structure is fired in asuitable temperature. The temperature used in the LTCC technique isaround 850° C. and in the HTCC technique around 1600° C. However, theproblem of microstrips, strip lines and coplanar conductors made withthese techniques is the high attenuation per unit of length, low powermargin and relatively low ElectroMagnetic Compatibility (EMC). Theseproblems limit the use of these conductor structures in the applicationswhere the above-mentioned properties are needed.

SUMMARY OF THE INVENTION

The objective of the invention is to accomplish a waveguide structureimplemented with multilayer ceramics, by which the above-mentioneddrawbacks of the prior art guide structure can be reduced.

The method according to the invention is characterized in that forcreating a waveguide in the direction of the z-axis:

-   -   at least two impedance change points in the direction of the yz        plane of the structure are formed in the structure to limit the        length a of the core of the waveguide in the direction of the        x-axis, and    -   that in the xz plane, the core of the waveguide is limited with        a first and a second layer of conductive material, which is silk        screen printed on top of the ceramic layers that form the core        of the waveguide, and which conductive planes are used to limit        the length b of the core of the waveguide in the direction of        the y-axis.

The waveguide according to the invention is characterized in that itcomprises:

-   -   the core part of the waveguide of the structure of the circuit        unit in the direction of the z-axis,    -   at least two points of impedance discontinuity in the yz-plane,        by which the length a of the core part of the waveguide has been        limited in the direction of the x-axis, and    -   a first and a second layer of conductive material in the xz        plane, by which layers the dimension b of the core part of the        waveguide has been limited in the direction of the y-axis.

The basic idea of the invention is the following: A waveguide fullyintegrated into the structure is manufactured with the multilayerceramic technique. The core part of the waveguide is made of dielectricmaterial with a suitable permittivity ε_(r), which is separated from therest of the ceramic structure in one plane by two layers of conductivematerial forming parallel planes, and in another plane, which isperpendicular to the previous planes, by two cavities filled with airand/or joining holes filled with conductive material.

The invention has the advantage that the waveguide can be manufacturedsimultaneously with other components manufactured with the multilayerceramic technique.

In addition, the invention has the advantage that the feedingarrangement of the waveguide can be implemented with the same multilayerceramic technique.

The invention also has the advantage that the manufacturing costs of awaveguide manufactured with the method are lower than those of awaveguide made of separate components and joined to the structure in aseparate step.

Furthermore, the invention has the advantage that it has a good EMCprotection as compared to a strip line, microstrip or coplanarconductor.

Other objects and features of the present invention will become apparentfrom the following detailed description considered in conjunction withthe accompanying drawings. It is to be understood, however, that thedrawings are intended solely for purposes of illustration and not as adefinition of the limits of the invention, for which reference should bemade to the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in more detail.Reference will be made to the accompanying drawings, in which

FIG. 1 shows a prior art, air-filled waveguide made of conductivematerial,

FIG. 2 shows an exemplary embodiment implemented with the multilayerceramic technique, in which the side walls of the waveguide are formedof cavities filled with air,

FIG. 3 shows another exemplary embodiment implemented with themultilayer ceramic technique, in which the side walls of the waveguideare formed of air-filled cavities and vias in the vicinity thereof,filled with conductive material,

FIG. 4 shows an example of a waveguide according to the secondembodiment of the invention implemented with the multilayer ceramictechnique as a section in the x-y plane,

FIG. 5 a shows an example of one way according to the invention toexcite a waveform capable of propagating in the waveguide according tothe first embodiment of the invention,

FIG. 5 b shows an example of another way according to the invention toexcite a waveform capable of propagating in the waveguide according tothe first embodiment of the invention,

FIG. 5 c shows an example of a third way according to the invention toexcite a waveform capable of propagating in the waveguide according tothe first embodiment of the invention,

FIG. 6 a shows an yz-plane presentation of one way of joining awaveguide according to an embodiment of the invention to a microstripconductor, and

FIG. 6 b shows an yz-plane presentation of fitting the feeding point ofa waveguide according to the invention to a waveguide.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 was presented in connection with the description of the priorart. In connection with the description of FIGS. 2 to 6, reference ismade to the directions of the axes x, y and z shown in FIG. 1. Thedirections of the axes are the same as those shown in the example ofFIG. 1, although the axes are not drawn in all the figures. The symbolε_(r) in this and the following figures refers to the particular valueof permittivity which the materials marked “ε_(r)” have, i.e. all theceramic material is labeled “ε_(r)” to indicate they all have the samepermittivity.

FIG. 2 shows an example of a waveguide according to the first embodimentof the invention, implemented with the multilayer ceramic technique. Thestructure shown in FIG. 2 is part of a larger circuit structureimplemented with the multilayer ceramic technique, which is not shown inits entirety in the drawing. The waveguide structure is surrounded onboth sides by the structures 21 and 27 shown in the drawing, whichconsist of several green tapes. The permittivity ε_(r) of the ceramicmaterial used in them is clearly higher than the permittivity of air,which is of the magnitude 1, as is well known. Other parts of thestructure, which are both above and below the waveguide structure shownin the drawing, viewed in the direction of the y-axis, consist mainly ofthe same ceramic material. The core part 23 of the waveguide consists ofthe same ceramic material as the rest of the circuit structure. Thewidth of the waveguide in the direction of the x-axis is limited byair-filled cavities 22 and 26 essentially in the direction of the yzplane. The interface of the air-filled cavity 22 or 26 forms adiscontinuity of the characteristic impedance against the core part 23in view of the electromagnetic wave front. This discontinuity of thecharacteristic impedance mainly reflects the wave front, which iscapable of propagating in the core part 23 of the waveguide, back to thecore part 23, while the wave front propagates in the direction of thez-axis. The waveguide is limited in the xz-plane by a first surface 24and a second surface 25, which are made of some conductive material andwhich form essentially parallel planes. These planar surfaces 24 and 25can be made either such that they completely cover the core part 23 orthey are partly gridded. These planar, conductive surfaces 24 and 25 canbe made, for example, of conductive pastelike material, by metallizingthe surfaces of the core part 23 in these planes or also by covering thecore part 23 by separate, thin, conductive filmy material.

In the waveguide according to the first embodiment of the invention, thelowest possible propagating waveform is the TEM(Transverse-electromagnetic) waveform, the electric or magnetic field ofwhich does not have a component in the direction of the z-axis of thedrawing. The cut-off frequency of this waveform is 0 Hz, as is known,which means that direct current can flow in the waveguide. A waveguideaccording to the first embodiment of the invention can also transmitother higher, possibly desired TE_(mn) or TM_(mn) (Transverse-magnetic)waveforms, the corresponding cut-off frequencies of which can becalculated according to the dimensioning rules of an ordinary waveguide,which dimensioning rules have been presented in connection with thedescription of FIG. 4.

FIG. 3 shows an example of a waveguide according to the secondembodiment of the invention. The structure shown in FIG. 3 is part of alarger structure implemented with the multilayer ceramic technique,which is not shown in its entirety in the drawing. The waveguidestructure is surrounded on both sides by the structures 31 and 37 shownin the drawing, which consist of several green tapes. The permittivityε_(r) of the ceramic material used in them is clearly higher than thepermittivity of air, which is of the magnitude 1. Other parts of thestructure, which are both above and below the waveguide structure shownin the drawing, viewed in the direction of the y-axis of the drawing,also consist mainly of the same ceramic material. The core part 33 ofthe waveguide consists of the same ceramic material as the rest of thecircuit structure. The width of the waveguide in the direction of thex-axis is limited by two essentially parallel impedance discontinuities,which are formed of via posts 38 and 39 in the direction of the y-axisof the drawing together with the air-filled cavities 32 and 36. Theair-filled cavities 32 and 36 have a similar construction as waspresented in connection with the description of the cavities shown inFIG. 2. The via posts 38, 39 are filled with conductive, pastelikematerial in connection with the manufacture of the circuit structure.When the LTCC technique is used, either AgPd paste or Ag paste can beused advantageously. If the waveguide structure according to theinvention is entirely surrounded from all sides by other ceramic layers,the cheaper Ag paste can be used. If part of the created waveguidestructure remains exposed to the external atmosphere, the more expensiveAgPd paste must be used. The via posts 38, 39 combine the essentiallyparallel first plane 34 and second plane 35, which are formed ofconductive material and which limit the core part 33 in the xz plane.

In the embodiment shown in FIG. 3, one via post 38 and 39 for each sideof the core part are shown in the drawing as viewed in the direction ofthe x-axis. The waveguide structure according to the invention can alsobe implemented by adding several similar via posts to the core part 33.It is also possible to add more similar via posts to the parts 31 and 37of the circuit structure behind the air cavities 32 and 36, whereby theEMC properties of the waveguide are further improved.

FIG. 4 shows an example of a structure according to the secondembodiment of the invention as a section in the xy plane. The ceramiccircuit structure is assembled by layers of ceramic plates/strips 41.The waveguide is separated from the rest of the structure in thedirection of the x-axis by air-filled cavities 42 and 46 in thedirection of the yz plane (not shown in FIG. 4), the width of whichcavities is the measure L shown in the drawing and the height is themeasure b shown in the drawing, and via posts 48 and 49 filled withconductive material. The core part 43 of the waveguide is formed byceramic material, the permittivity ε_(r) of which is high compared toair. The width of the core part of the waveguide in the direction of thex-axis in denoted by the letter a in the drawing. The width L of theair-filled cavities 42 and 46 in the x-plane is selected such that itsmagnitude corresponds to a fourth of the wavelength of the cut-offfrequency f_(c). Then the waveguide structure emits as littleinterference radiation as possible to its environment. In the xz plane(not shown in FIG. 4), which is perpendicular to the surface shown inFIG. 4, the waveguide is limited by a first plane 44 and a second plane45, which are essentially parallel and made of conductive material. Thefirst plane 44 and the second plane 45 are connected to each other byvias 48 and 49, which are filled with conductive material. The waveformsTE_(mn) and TM_(mn) can propagate in a waveguide according to theembodiment shown in the drawing. The cut-off frequencies f_(cmn) ofthese waveforms are obtained from the known formula:$f_{{cm},n} = {\frac{1}{2\sqrt{\mu ɛ}}\sqrt{\left( \frac{m}{a} \right)^{2} + \left( \frac{n}{b} \right)^{2}}}$

In the formula, the indexes m and n refer to the number of maximums inthe direction of the x and y axes of the transverse field distributionof the TE_(mn) or TM_(mn) waveform, measure a denotes the width of thewaveguide in the direction of the x-axis, and measure b denotes theheight of the waveguide in the direction of the y-axis. The terms μ andε in the formula are the permeability and permittivity values of theceramic material of the core part 43 of the waveguide.

FIGS. 5 a, 5 b and 5 c show three different examples of how the desiredwaveform can be excited in waveguides according to the invention. Thewaveguide used in the examples of the figures is a waveguide accordingto the first embodiment, but the solutions function in accordance withthe same principle in waveguide structures according to the secondembodiment of the invention as well.

In the example of FIG. 5 a, the core 53 a of the waveguide is separatedfrom the rest of the circuit structure, which is represented by parts 51a and 57 a of the structure in the drawing, by air-filled cavities 52 aand 56 a and a first plane 54 a and a second plane 55 a, which areessentially parallel and made of conductive material. In order to excitethe desired waveform, a hole 58 a has been made at the desired point inthe first plane 54 a of the waveguide. When a radiating element, whichis not shown in the drawing, is placed in the vicinity of the hole 58 a,the result is that part of the field radiated by the element istransferred through the hole 58 a to the waveguide according to theinvention. The radiating element can be any circuit element capable ofradiating, or possibly another waveguide according to the invention, inthe wall of which a hole of corresponding shape and capable of radiatinghas been made. By selecting the radiating frequency correctly, anelectromagnetic waveform of the desired kind and capable of propagatingcan be excited in the waveguide.

FIG. 5 b shows another possible way of exciting a waveform capable ofpropagating in a waveguide according to the invention. In the example ofFIG. 5 b, the core 53 b of the waveguide is separated from the rest ofthe circuit structure, which is represented in the drawing by parts 51 band 57 b, by air-filled cavities 52 b and 56 b and a first plane 54 band a second plane 55 b, which are essentially parallel and made ofconductive material. In order to excite the desired waveform, there is ahole 58 b made at the desired point of the conductive first plane 54 b,and the hole is fitted with a cylindrical probe 59 b leading to the corepart 53 b of the waveguide.

The probe is preferably made of the same conductive material as theplanar first surface 54 b and second surface 55 b of the waveguide. Theprobe 59 b is connected to the desired signal inputting conductor in thecircuit structures above the planar first surface 54 b. The signalconductor can be a strip line or a microstrip, for example. Theconductor and other circuit structures above are not shown in FIG. 5 b.

FIG. 5 c shows a third possible way of exciting a waveform capable ofpropagating in a waveguide according to the invention. In the example ofFIG. 5 c, the core 53 c of the waveguide is separated from the rest ofthe unit, which is represented in the drawing by parts 51 c and 57 c, byair-filled cavities 52 c and 56 c and a first plane 54 c and a secondplane 55 c, which are essentially parallel and made of conductivematerial. In order to excite the desired waveform in the waveguide,there is a hole 58 c made at the desired point of the first plane 54 cmade of conductive material, and the hole is fitted with a coupling loop59 c leading to the core part 53 c of the waveguide. The coupling loop59 c is connected to the desired signal inputting conductor in thecircuit structures above the planar first surface 54 c. The signalconductor can be, for example, a stripline, microstrip or a coplanarconductor. The signal inputting conductor and other circuit structuresabove are not shown in FIG. 5 c. The coupling loop 59 c is manufacturedof conductive material in connection with the manufacture of the rest ofthe circuit structure implemented with the multilayer ceramic technique.

FIG. 6 a shows, by way of example, how the microstrip and the waveguideaccording to the invention can be joined together. The figure shows asection in the yz plane of the point where the conductors are connected.The circuit structure has been implemented by joining together severallayers of ceramic plates 61 a. The portion of the microstrip 60 a isformed by the signal conductor 63 a (labeled “S” in FIG. 6 a) and theground conductor 62 a (labeled “G” in FIG. 6 a). The impedance of thetransmission line changes at the point where the microstrip and thewaveguide 68 a are joined together. High impedance mismatches cause anundesired reflection of the signal back to its incoming direction in theabove-mentioned interface. This reflection problem can be diminished bymaking at the joint a special structure, in which the impedance level ofthe transmission line is gradually changed. In the example of FIG. 6 a,this matching of the impedances has been implemented by a so-calledquarter-wave transformer 67 a It consists of steplike changes of thewaveguide geometry of the length of λ/4 in the direction of the z-axisin the drawing. In FIG. 6 a, it is accomplished by means of conductiveplane surfaces 66 a, which are connected to each other in the directionof the y-axis by vias 64 a made of conductive material. In the directionof the x-axis, these planes 66 a reach across the whole core part of thewaveguide. The second plane 65 a forms the lower surface of thewaveguide. The electric properties of the ceramic material used in thestructure are similar in all parts of the circuit structure in theexample of the drawing.

FIG. 6 b shows an example of another way of joining a waveguideaccording to the invention to another electric circuit. The figure showsa section in the yz plane of the point where the transmission lines areconnected. The circuit structure of the component has been implementedby joining together several layers of ceramic plates 61 b. The excitingsignal is brought to the waveguide by means of a cylindrical probe 63 b.In the example of the drawing, the probe comes to the waveguide 68 bthrough the first plane 62 b, which forms the upper surface of thewaveguide, and a hole 69 b made in the plane. Thus the probe 63 b doesnot have a galvanic connection to the conductive first plane 62 b. Theprobe 63 b itself may reach through several ceramic circuit structuresin the direction of the y-axis of the drawing, when required. Theimpedance mismatch created at the feeding point of the signal is reducedby a quarter-wave (λ/4) transformer 67 b of the kind described inconnection with FIG. 6 a. The quarter-wave (λ/4) transformer 67 bconsists of conductive plane surfaces 66 b, which are connected to eachother in the direction of the y-axis of the drawing by vias 64 b made ofconductive material. In the direction of the x-axis of the drawing,these planes 66 b reach across the whole core part of the waveguide. Thesecond plane 65 b forms the lower surface of the waveguide. The electricproperties of the ceramic material used in the structure are in similarin all parts of the circuit properties of the ceramic material used inthe structure are similar in all parts of the circuit structure in theexample of the drawing.

Calculatory simulations have been performed on the embodiments of thewaveguides according to the invention. The simulations have beenperformed on both embodiments according to the invention with the samestructural dimensions, whereby the measure a of the core part of thewaveguide has been 5 mm, measure b 2 mm, ε_(r) of the ceramic material5.9 and the measure L in the direction of the x-axis of the air-filledcavities that are part of the waveguide structure 2.5 mm. A mode ofoperation according to TE₁₀ has been used in the simulation, and thefrequency used has been 18 GHz. As a result of the simulation, the firstembodiment according to the invention had an attenuation of 1.7 dB/cm.With the same structural dimensions a and b and the same frequency 18GHz, the waveguide structure according to the second embodiment of theinvention had an attenuation value of 0.7 dB/cm.

Some preferred embodiments of the invention have been described above.However, the invention is not limited to the solutions described above.The inventive idea can be applied in many different ways within thescope defined by the attached claims.

Thus, while there have been shown and described and pointed outfundamental novel features of the present invention as applied topreferred embodiments thereof, it will be understood that variousomissions and substitutions and changes in the form and details of thedevices described and illustrated, and in their operation, and of themethods described may be made by those skilled in the art withoutdeparting from the spirit of the present invention. For example, it isexpressly intended that all combinations of those elements and/or methodsteps which perform substantially the same function in substantially thesame way to achieve the same results are within the scope of theinvention. Substitutions of elements from one described embodiment toanother are also fully intended and contemplated. It is also to beunderstood that the drawings are not necessarily drawn to scale but thatthey are merely conceptual in nature. It is the intention, therefore, tobe limited only as indicated by the scope of the claims appended hereto.

1. A method for manufacturing a waveguide in a circuit structure using a multilayer ceramic technique, wherein said circuit structure is assembled of separate layers of ceramic, said ceramic having a permittivity ε_(r) which is higher than the corresponding value of air, and wherein, in said multilayer ceramic technique, layers, cavities, and holes are made in the ceramic layers, said method comprising the steps of: forming two air-filled channels in said layers of ceramic extending the length of the waveguide, wherein a core of the waveguide is defined between said two air-filled channels; forming by silk screen printing essentially parallel first and second planes of conductive material above and below the core of the waveguide, wherein said first and second conductive planes define a top and a bottom of the core of the waveguide, and wherein said first and second conductive planes do not extend past said two air-filled channels; and completing the circuit structure including the waveguide by exposing the circuit structure to a heat treatment; wherein the multilayer ceramic technique is one of High Temperature Cofired Ceramics (HTCC) and Low Temperature Cofired Ceramics (LTCC).
 2. A method for manufacturing a waveguide in a circuit structure using a multilayer ceramic technique, wherein said circuit structure is assembled of separate layers of ceramic, said ceramic having a permittivity ε_(r) which is higher than the corresponding value of air, and wherein, in said multilayer ceramic technique, layers, cavities, and holes are made in the ceramic layers, said method comprising the steps of: forming two air-filled channels in said layers of ceramic extending the length of the waveguide, wherein a core of the waveguide is defined between said two air-filled channels and a width of each of the two air-filled channels is substantially one-fourth of a wavelength of a cutoff frequency of the waveguide; and forming by silk screen printing essentially parallel first and second planes of conductive material above and below the core of the waveguide, wherein said first and second conductive planes define a top and a bottom of the core of the waveguide, and wherein said first and second conductive planes do not extend past said two air-filled channels; and completing the circuit structure including the waveguide by exposing the circuit structure to a heat treatment.
 3. A waveguide manufactured using a multilayer ceramic technique comprising: a waveguide core defined by: two air-filled channels extending the length of the waveguide; a bottom surface of conductive material under the waveguide core; and a top surface of conductive material on the waveguide core; wherein said top and bottom surfaces are substantially parallel planes; wherein said top and bottom surfaces do not extend past said two air-filled channels; and two remaining waveguide portions defined outside said two air-filled channels; wherein the waveguide core and the two remaining portions comprise ceramic material having the same permittivity, and wherein said permittivity is greater than the permittivity of air.
 4. The waveguide according to claim 3, wherein said waveguide core further comprises: at least one row of vias filled with conductive material and positioned close to at least one of the air-filled channels, whereby said vias galvanically connect said top and bottom surfaces.
 5. The waveguide according to claim 3, wherein a hole in disposed in the top surface of conductive material to thereby excite an electromagnetic field intended to propagate in the waveguide core.
 6. The waveguide according to claim 3, wherein a hole is disposed in the top surface of conductive material, and wherein said hole is fitted with a probe leading to the waveguide core to thereby excite an electromagnetic field intended to propagate in the waveguide.
 7. The waveguide according to claim 3, wherein a hole is disposed in the top surface of conductive material, and wherein said hole is fitted with a coupling loop leading to the waveguide core to thereby excite an electromagnetic field intended to propagate in the waveguide.
 8. The waveguide according to claim 3, wherein an interface between the waveguide core and air in the two air-filled channels defines a discontinuity of the characteristic impedance of the waveguide core.
 9. The waveguide according to claim 3, wherein a ceramic structure including the waveguide is comprised substantially of the same ceramic material.
 10. The waveguide according to claim 3, wherein the substantially parallel top and bottom surfaces on the waveguide core either substantially cover the waveguide core or (ii) are partly gridded.
 11. The waveguide according to claim 3, wherein the multilayer ceramic technique is one of High Temperature Cofired Ceramic (HTCC) and Low Temperature Cofired Ceramics (LTCC).
 12. The waveguide according to claim 3, wherein a width of each of the two air-filled channels is substantially one-fourth of a wavelength of a cutoff frequency of the waveguide.
 13. The waveguide according to claim 3, wherein a waveform that can propagate in the direction of the length of the waveguide is one of a transverse-electric and transverse-magnetic waveform.
 14. A method for manufacturing a waveguide in a circuit structure using a multilayer ceramic technique, wherein said circuit structure is assembled of separate layers of ceramic, said ceramic having a permittivity ε_(r) which is higher than the corresponding value of air, and wherein, in said multilayer ceramic technique, layers, cavities, and holes are made in the ceramic layers, said method comprising the steps of: forming two air-filled channels in said layers of ceramic extending the length of the waveguide, wherein a core of the waveguide is defined between said two air-filled channels; forming by silk screen printing essentially parallel first and second planes of conductive material above and below the core of the waveguide, wherein said first and second conductive planes define a top and a bottom of the core of the waveguide, and wherein said first and second conductive planes are defined between said two air-filled channels; forming a first row of vias in the core of the waveguide, wherein said first row of vias is positioned close to a first air-filled channel of the two air-filled channels; forming a second row of vias in the core of the waveguide, wherein said second row of vias is positioned close to a second air-filled channel of the two air-filled channels; forming a third row of vias in the core of the waveguide; and completing the circuit structure including the waveguide by exposing the circuit structure to a heat treatment; wherein each via is filled with conductive material whereby first and second planes of conductive material are galvanically connected.
 15. A method for manufacturing a waveguide in a circuit structure using a multilayer ceramic technique, wherein said circuit structure is assembled of separate layers of ceramic, said ceramic having a permittivity ε_(r) which is higher than the corresponding value of air, and wherein, in said multilayer ceramic technique, layers, cavities, and holes are made in the ceramic layers, said method comprising the steps of: forming two air-filled channels in said layers of ceramic extending the length of the waveguide, wherein a core of the waveguide is defined between said two air-filled channels; forming by silk screen printing essentially parallel first and second planes of conductive material above and below the core of the waveguide, wherein said first and second conductive planes define a top and a bottom of the core of the waveguide, and wherein said first and second conductive planes are defined between said two air-filled channels; and forming a quarter-wave transformer at an end of the waveguide core where a signal is fed into the waveguide core; and completing the circuit structure including the waveguide by exposing the circuit structure to a heat treatment.
 16. A method for manufacturing a waveguide in a circuit structure using a multilayer ceramic technique, wherein said circuit structure is assembled of separate layers of ceramic, said ceramic having a permittivity ε_(r) which is higher than the corresponding value of air, and wherein, in said multilayer ceramic technique, layers, cavities, and holes are made in the ceramic layers, said method comprising the steps of: forming two air-filled channels in said layers of ceramic extending the length of the waveguide, wherein a core of the wavelength is defined between the two air-filled channels and two remaining portions of ceramic material are defined outside the two air-filled channels; forming by silk screen printing essentially parallel first and second planes of conductive material above and below the core of the waveguide, wherein said first and second conductive planes define a top and a bottom of the core of the waveguide, and wherein said first and second conductive planes are defined between said two air-filled channels; forming at least one row of vias in one of the two remaining portions of ceramic material; and completing the circuit structure including the wavelength by exposing the circuit structure to a heat treatment.
 17. A method for manufacturing a waveguide using a multilayer ceramic manufacturing technique, comprising the steps of: forming two air-filled channels extending the length of the waveguide, whereby a waveguide core is defined between said two air-filled channels and two remaining waveguide portions are defined outside said two air-filled channels, wherein the waveguide core and the two remaining waveguide portions comprise ceramic material having the same permittivity, and wherein said same permittivity is greater than the permittivity of air; forming a bottom surface of conductive material under the waveguide core, wherein said bottom surface does not extend over the remaining waveguide portions; and forming a top surface of conductive material on the waveguide core, wherein said top surface does not extend over the remaining waveguide portions, wherein said top and bottom surfaces are substantially parallel planes.
 18. The waveguide manufacturing method according to claim 17, further comprising the steps of: forming a first row of vias in the waveguide core, wherein said first row of vias is positioned close to a first air-filled channel of the two air-filled channels; and forming a second row of vias in the waveguide core, wherein said second row of vias is positioned close to a second air-filled channel of the two air-filled channels.
 19. The waveguide manufacturing method according to claim 18, further comprising the step of: forming a third row of vias in the core of the waveguide.
 20. The waveguide manufacturing method according to claim 17, further comprising the step of: forming a quarter-wave transformer at an end of the waveguide core where a signal is fed into the waveguide core.
 21. The waveguide manufacturing method according to claim 17, further comprising the step of: forming at least one row of vias filled with conductive material and positioned close to at least one of the air-filled channels, whereby said vias galvanically connect said top and bottom surfaces.
 22. The waveguide manufacturing method according to claim 17, further comprising the step of: disposing a hole in the top surface of conductive material by means of which an electromagnetic field can be excited to thereby propagate in the waveguide core.
 23. The waveguide manufacturing method according to claim 22, further comprising the step of: fitting a probe in said hole, wherein said probe excites the electromagnetic field.
 24. The waveguide manufacturing method according to claim 22, further comprising the step of: fitting a coupling loop in said hole leading to the waveguide core, wherein said coupling loop excites the electromagnetic field.
 25. The waveguide manufacturing method according to claim 17, wherein an interface between the waveguide core and air in the two air-filled channels defines a discontinuity of the characteristics impedance of the waveguide core.
 26. The waveguide manufacturing method according to claim 17, wherein a ceramic structure including the waveguide is comprised substantially of the same ceramic material.
 27. The waveguide manufacturing method according to claim 17, wherein the substantially parallel planes of conductive material comprising the top and bottom surfaces on the waveguide core either (i) substantially cover the waveguide core or (ii) are partly gridded.
 28. The waveguide manufacturing method according to claim 17, wherein the multilayer ceramic technique is one of High Temperature Cofired Ceramics (HTCC) and Low Temperature Cofired Ceramics (LTCC).
 29. The waveguide manufacturing method according to claim 17, wherein a width of each of the two air-filled channels is substantially one-fourth of a wavelength of a cutoff frequency of the waveguide.
 30. The waveguide manufacturing method according to claim 17, wherein a waveform that can propagate in the direction of the length of the waveguide is one of a transverse-electric and transverse-magnetic waveform.
 31. The waveguide manufacturing method according to claim 17, further comprising the steps of: forming at least one row of vias in the core of the waveguide, wherein said at least one row of vias is positioned close to at least one of the air-filled channels and each via in the at least one row of vias is filled with conductive material whereby said first and second planes of conductive material are galvanically connected. 